While a liquid crystal display (LCD) has been proliferating as the most popular type of a flat panel display, the category of a non-light emitting type display includes, besides LCD, an electrochromic display (ECD). The category of a light emitting type flat panel display includes a plasma display panel (PDP) and an electro-luminescent display (ELD) which are in the limelight recently. Among various types of electro-luminescent displays, an organic electro-luminescent display has the capability of high luminance display and full-color display, and has been researched particularly vigorously.
These flat panel displays are driven by applying a voltage across opposing first and second electrodes, or flowing a current therethrough. Since the electric field tends to concentrate at edges of the electrode where radius of curvature is small, undesirable phenomena are likely to occur at the edges such as dielectric breakdown and leakage current.
As means for preventing such phenomena, it is known to cover the edges of the first electrode with an insulating layer (also referred to as a separator), which mitigates the field concentration at the edges of the electrode. Polyimide resin, novolak resin, acrylic resin or the like is commonly used to form the insulating layer. Each of these resins has variations having different properties of being non-photosensitive, negatively photosensitive and positively photosensitive. Since the use of a photosensitive resin enables it to form the insulating layer in a desired pattern without using a photoresist, complicated production process can be simplified and poor yield of production can be improved. In a conventional process for producing the insulating layer, however, there has been such a problem that transcribed trace is left on the insulating layer thereby deteriorating the product quality. The transcribed trace refers to a visible surface unevenness caused on a heated object by an object that approached or made contact with the heated object.
For example, when a heated object comprising a supporting substrate such as glass substrate coated with a resin composition is dried by heating with a hot plate, the heated object is held on a fixture such as proximity pins and the heated object is heated by the heat of radiation from the hot plate. In this process, transcribed trace often appears at the place where the proximity pins touched. The transcribed trace exerts an adverse influence on the subsequent processes such as exposure and development, and eventually causes such problems as film thickness uniformity of the insulating layer after curing. The transcribed trace appears particularly frequently in polyimide and/or a polyimide precursor resin which are widely used to form the insulating layer.
Methods proposed to prevent the occurrence of transcribed trace include one based on the modification of the heating apparatus. Japanese Patent Laid-Open No. 8-236431 discloses a method wherein a second hot plate that can be moved up and down is provided so as to make contact over the entire surface of a substrate which is to be heated, thereby to eliminate localized temperature gradient. Japanese Patent Laid-Open No. 8-279548 and Japanese Patent Laid-Open No. 2002-40223 disclose such methods that use proximity pins made of a synthetic resin such as polyimide and restrict the contact between the proximity pins and the heated object within an area as small as 0.008 to 0.2 mm2, or form the proximity pins in a tubular shape so as to decrease the heat conductivity.
However, these techniques suppress the occurrence of transcribed trace by modifying the heating apparatus, thus making it necessary to make specific modification for each heating apparatus which is applied, and are therefore expensive and time-consuming. Thus there have been demands for a photosensitive resin composition which is less likely to cause transcribed trace even when a special heating apparatus is not used.